1. Field of Invention
This invention relates to the field of portable communication transceivers and specifically to a portable transceiver antenna system which is adapted for electronically altering the antenna/receiver characteristics in response to changing environments in which the transceiver is utilized.
2. Description of the Prior Art
Portable radio transceivers are used in several facets of communications technology. Portable transceivers can be found in paging systems, two-way communication systems such as those used by police and other public servants, and more recently portable receivers have found use in portable data terminals which are used to communicate with a host computer over a radio link. Portable data terminals provide computer diagnostics and expanded computer power in the portable terminal.
Typical portable paging receivers in the past have incorporated loop-type antennas which are typically responsive to the magnetic field component of a transmitted RF signal. This type of antenna system provides marginal performance, but is desirable because the loop antenna can be concealed within a radio housing and responds well when the receiver is in close proximity to a human body. However, the overall sensitivity of the antenna/receiver combination is somewhat degraded where the receiver is not in close proximity to a human body. In addition, loop-type antennas exhibit certain problems which make them undesirable for use in a portable device which also utilizes a transmitter.
Portable units which contain both a receiver and transmitter are utilized in public service and private industry where communication between a number of units is desirable. The units typically contain a receiver and transmitter used in combination with an external helical, monopole or dipole antenna. These antennas are responsive to the electrical field of an RF signal and provide superior reception and transmission characteristics, when the antenna is located externally to the portable device housing. In addition, helical and monopole antenna performance is degraded substantially when the antenna is operated in close proximity to a human body.
A paging receiver is normally worn on the body when in use. Unlike the pager, a portable transceiver is utilized in many different environments. It may be handheld in front of the face, worn on the hip, or placed on a conducting or non-conducting surface.
With the advance of data processing and data communication techniques, it is now desirable to manufacture portable communication devices which are capable of transmitting and receiving data in cooperation with a host computer. These devices are for instance useful to servicemen who use the portable terminal to perform on-site computer testing and diagnostics, and to keep records thereof. It is desirable to manufacture a portable data terminal as small as possible, so the terminal can be transported easily. In addition, it is also desirable to provide a portable data terminal with an antenna system which is completely enclosed within the terminal housing. This feature enhances the portability of the data terminal, as well as providing protection for the terminal antenna system.
These terminals may be carried in the hand, worn on the hip, put in a coat pocket, used on a desk, or put into a service tool kit. The orientation of the unit, as well as its physical environment is constantly changing as it is utilized from one time to the next.
Since the portable data terminal is operated in a number of distinct environments and orientations, it is necessary to provide an antenna system which is adaptable to a changing environment. Adaptability is especially useful when the portable data terminal is used in close proximity to a human body.
Adaptable antenna systems exist in the prior art but have primarily focused on antenna systems which are used in mobile or fixed communications systems. These systems are typically implemented in an automobile-based environment and utilize two identical antennas which are placed so as to be more than a quarter wavelength from each other. When the antenna system passes through a changing environment such as an automobile passing a building, one antenna may encounter a null or a point in space where the Rf energy is low due to multipath interference, and the radio receiver will select the alternate antenna.
These adaptable antenna systems are known as mobile diversity systems, and operate on a principle which provides that if a null occurs at a point in space, the likelihood of a null occurring at a point some distance away from the null, typically one quarter wavelength, is low. Simply stated, if one antenna in the mobile diversity system is in a null, the other antenna is probably not in a null.
Furthermore, mobile diversity systems typically utilize identical antennas which are oriented in the same plane and exhibit identical reception characteristics. The antennas in a mobile diversity system are located externally to the vehicle and typically utilize monopole, dipole, or helical antennas.
This type of antenna system is designed to operate in a free-space environment where the electromagnetic waves exhibit 377 ohms impedance, and the immediate physical surrounding of the antennas do not change. It would not particularly be advantageous on a portable tranceiver when the the environment changes, for example being in close proximity to the human body.
Adaptable antenna systems have not been developed for portable communication devices in part due to conventional teachings which indicate that a diversity type antenna system requires antennas which are placed at least one quarter wavelength apart, which is usually not possible in a portable communication device. In addition, it has been neither practical nor desirable to place a plurality of monopole type antennas external to a radio housing due to space limitations and aesthetics.
Portable radio's are typically provided with monopole antennas which are necessarily larger than a radio housing because of the range of operating frequencies of the unit. This is especially true for radio's which transmit as well as receive. A radio housing limits the placement as well as the size of the antenna, whether the antenna is disposed internal or external to the housing. Finally, relatively minor alterations to the radio housing can cause drastic changes in the operating characteristics of the radio unit. The most advanced radios of today may be provided with a single enclosed antenna and the enclosed antennas of today are not capable of the transmitting an RF signal. Techniques have not previously been developed which provide for the placement of an efficient multi-antenna system within a radio housing.
For the foregoing and other shortcomings, there is a need for an improved adaptable antenna system for use with a portable transceiver in which the antenna system can be used to transmit or reliably receive a signal subject to deep fades while at the same time being contained completely within the radio housing.
The present invention overcomes the foregoing problems in a portable receiver by incorporating a plurality of antennas within a radio housing, they may or may not be disposed a quarter wavelength apart from each other, and by relying on antenna characteristics and placement they provide a diverse antenna selection within the system. In particular, they are designed each one to respond best in a different type of physical environment. They designed and placed so as to minimize the probability of all being degraded simultaneously during typical use of the portable receiver.